1. Field of the Invention
This invention relates to a synthetic string for sporting applications such as tennis, badminton, racquetball and squash racquets or the like.
2. Description of the Prior Art
Racquet strings generally come in a variety of nominal diameter sizes (gauge) and are tensioned between 10 to 85 pounds, the string gauge and the tension depending upon the size of the racquet, the style of play and preference of the player. Conventional racquets are basically strung with either two-piece strings or one piece string, the latter being preferable since only two knots rather than four knots are required to tie the ends of the string. Conventional racquet strings have two string components, main-strings running generally parallel to the length-wise direction of the racquet and cross-strings running perpendicularly to the main-strings. In stringing a conventional stringing pattern, usually all of the main strings are positioned and tensioned first and then each cross-string is woven through the main string and tensioned. The cross-strings in general are interwoven alternately with the main-strings to form an interwoven mesh-like pattern.
The performance of a string is categorized in several ways. The three most important performance categories are playability, durability and tension loss. In prior strings, there was always a tradeoff between a highly playable string which sacrificed durability and a highly durable string which sacrificed playability. One example of a highly playable string which sacrifices durability is a natural gut string from sheep, cow, whale, and others. A natural gut string plays well because it is highly elastic (low in static stiffness) and highly resilient (low in dynamic stiffness). Elasticity is defined as the ability of a material to return to its original dimensions after the removal of stresses. Resilience is defined as the potential energy stored up in a deformed body. A natural gut string, however, is very sensitive to humidity, causing the string to either break or lose tension sooner and is highly susceptible to fraying (peeling) from abrasion, particularly at the string crossover locations, wearing the string rapidly.
An example of a highly durable string, but with less than average playability is a synthetic string which incorporates a highly abrasion resistant fiber such as para-aramids (KEVLAR, TECHNORA, TWARON), melt spun liquid crystal polymers (VECTRAN) and high molecular weight polyethylene (SPECTRA). These materials are highly abrasion resistant. However, they are also extremely stiff and inelastic, undesirably increasing the overall dynamic and static stiffnesses of the string, which contributes to a board-like feel which diminishes playability.
There are three modes of wear on a string. In the first mode, the rubbing action of the main-string over and against short lengths of the cross-strings creates notches in the main-strings. During play, particularly in tennis, the ball is usually hit with some degree of spin, the degree of spin depending on the particular shot being made, the style of the player and the string gauge, texture and spacing. Normally, to generate a spin on the ball, the string is brushed, in the direction parallel to the cross-strings and thus perpendicular to the main-strings, against the fuzzy, rough surface of the ball which imparts a tangential force on the ball and causes the main strings to slide over and rub against the cross strings. Rough textured strings generally impart more spin to the ball since the higher surface friction tends to bite into the ball better. Generally the greater the spin imparted to the ball, the greater the force will be placed on the main-strings, in the perpendicular direction thereof, forcing the main-strings to rub against the cross-strings. Specifically, since the ball is brushed parallel to the cross-strings, the cross-strings remain substantially stationary while the main-strings slide across the cross-strings. Thus, the cross-strings can be envisioned as a stationary knife or saw-like instrument cutting through the main-strings each time the main-strings move across the cross-strings.
All main-strings begin to experience notching to some degree in the outer coating and/or wraps thereof as one string rubs against another. The notching initially cuts through the outer coating or outer wraps and into the center core until the string prematurely breaks. See FIGS. 5, 5a. The primary reason for string breakage is due to the notch cutting into the core.
The second mode of wear occurs from the actual rubbing friction the ball creates during contact directly with the string surface. This is most pronounced on the top portion of the string where the intersections of the main- and cross-strings are created in a woven string mesh. See FIGS. 5, 5b.
The third mode of wear occurs on the stationary cross-string as the main-string slides across it. The rubbing friction of the notched area of the main-string over the length of the rubbing contact thereof with the cross-strings causes the cross-string to be gradually worn down. See FIGS. 5, 5c.
Wide-body racquets are the latest trend in the tennis world. With the advent of wide-bodies, a stronger and more durable string, able to withstand extreme string abrasion is needed. Wide-body racquets are extremely rigid and thus bend very little on impact, forcing the string-bed to work harder. The string has to work harder since there is no give or deflection in the racquet to absorb the energy imparted by the ball. Therefore, more energy is transferred to the string, causing greater loads on the strings and string intersections. As a result, string notching and premature string failure occurs more rapidly with wide-bodies. There is a great need, with the advent of wide-bodies, for a more durable string that is also playable.
Attempts have been made in the past to alleviate the notching problem. For example, U.S. Pat. No. 3,921,979 contemplates placing a small, self-lubricating plastic cross guide between each intersection of the main-strings and the cross-strings. However, the guides of the type contemplated in U.S. Pat. No. 3,921,979 are inconvenient and do not work well because they fall off the string with use, due to the impact. Moreover, the extraneous mass of the guides can also cause undesired vibrations. For these reasons, the guides of the type described in U.S. Pat. No. 3,921,979 have not been successful.
U.S. Pat. No. 4,238,262 issued to Fishel contemplates coating the intersection of the cross-strings and the main-strings with elastic adhesive to form a bond therebetween to prevent the strings from moving relative to each other. Although bonding strings together will alleviate the notching problem in the main-strings, the disadvantage to this is that if the strings are effectively bonded, their playability will be substantially degraded due to the adhesive interacting with the strings. Strings that are bonded at their intersection tend to feel "board-like" because the bonding at the intersection has the effect of stiffening the string-bed.
U.S. Pat. No. 4,377,620 discloses synthetic or natural gut strings which are coated with a coating film of minute particles of ethylene tetrafluoride. The particles are of a size ranging from 0.1 to 10 microns and are applied either from a dispersion in a solvent which is allowed to dry, or from a molten vehicle which is allowed to harden. The final string has only discontinuously spaced particles of the ethylene tetrafluoride in a thickness of the order of approximately 20 microns. As a result, the particles wear away quickly and thereafter the problem of notching and tension loss can ensue. Thus, the coating film of minute particles taught by this patent gives only temporary and limited protection against string wear.
Many types of racquet string construction have been contemplated in the past in attempting to produce strings that are durable and have a good playability. Some incorporate a durable abrasion resistant material of aramid polymer generically known as KEVLAR which is poly (paraphenylene terephthalamide), to form a durable, notch resistant string. KEVLAR material has excellent abrasion resistance. However, because KEVLAR material is relatively inelastic and has a very low resiliency, strings incorporating this material generally play very "board-like" and thus lack playability. In another instance, U.S. Pat. No. 4,530,206 shows a tennis racquet string incorporating twisted KEVLAR material in combination with a glass fiber as a core of the string, the elasticity of the string being not more than 5% at its maximum loading capacity.
In other types of string sold under the names of Endurance by Prince Manufacturing Inc. and Twaron by Head Sports, Inc., a nylon core is wrapped with a ribbon-like helical wrap of para-aramid fibers, the Prince string having a KEVLAR wrap and the Head Sports string having a TWARON wrap which is a KEVLAR type aramid fiber. The purpose of the wrap is to shield the core with an abrasion resistant material. Again, while KEVLAR/TWARON material has excellent wear characteristics, it is generally not a preferred material for a racquet string because the relatively inelastic characteristic of KEVLAR/TWARON material constrains the nylon core from stretching, causing the overall string to be less elastic and resilient (higher static and dynamic stiffness).
U.S. Pat. No. 4,391,088 contemplates a composite gut string which incorporates a highly resilient (low dynamic stiffness) gut center core reinforced with a protective jacket of highly inelastic (high static stiffness) KEVLAR material. The gut core is shielded with braided KEVLAR fibers. The reinforced core is then coated with polyurethane resin to seal the string. In essence, this string has a very low dynamic stiffness core encased in a very high dynamic stiffness KEVLAR sheath. Under tension, the sheath of the string would predominate as the load bearing element over the center core being loaded. Although durability will increase, the playability will suffer greatly due to the fact that the inelastic and nonresilient characteristics of the KEVLAR sheath would dominate.
Wilson Sporting Goods Company has marketed a tennis string called DUALTEC 137 which is similar to the performance of the string set forth in U.S. Pat. No. 4,391,088, in that a relatively low dynamic stiffness core is wrapped or surrounded by a very high dynamic stiffness aramid fiber known as TECHNORA, which is co-poly-(paraphenylene/3,4'-oxydiphenylene terephthalamide). Specifically, a pair of ribbon-like wraps of TECHNORA is spirally wrapped around a nylon core in opposite directions at 180.degree. apart. Due to the fact that TECHNORA material has a very high dynamic stiffness and is very inelastic, much like KEVLAR, it is generally not a preferable material for constructing a racquet string.
U.S. Pat. No. 4,568,415 shows a method of manufacturing a string which features a pair of ribbon-like wraps that are helically wound around a continuous core, similar to the wraps of DUALTEC 137. The disclosure relating to the manner in which the ribbon-like wraps are helically wound around the center core is incorporated herein by reference. The helically wound wraps of this patent are made of plastic, preferably olefins of high molecular weight and polyethylene/polypropylene/diene terpolymers of high molecular weight. The wraps made from these materials are relatively elastic in comparison to the KEVLAR material, but they are not as abrasion resistant and thus have little capability of preventing or retarding the notching from cutting into the core.
U.S. Pat. No. 4,275,117 discloses a string resulting from the integration of a thermoplastic sheath with a thermoplastic braided core of a different melting point under heat. By using a high melting sheath and a low melting core, the core can be melted into the sheath. Conversely, by using a low melting sheath and a high melting core, the sheath can be melted into the core. Additionally, a relatively high melting spiral wrap can be applied around the integrated core and sheath. Under heat, the spiral wrap is integrated into the sheath/core. Nylon 66 having a melting point of approximately 480.degree. F. is given as an example of the higher melting point thermoplastic material. A nylon terpolymer having a melting point of approximately 310.degree. F. and nylon 12 having a melting point of approximately 350.degree. F. are given as examples of the lower melting point thermoplastic material. The wraps made of the material set forth in this patent are made of relatively low melting point materials which have limited capacity to withstand the instantaneous frictional heat and temperature increase induced therein during ball impact on the strings. Thus, these relatively low melting point materials have limited effectiveness in preventing or retarding notching from cutting into the core.
U.S. Pat. No. 4,016,714 discloses a string formed by twisting a plurality of single strands to form a core and then forming an outer thermoplastic shell. In addition, to strengthen the string, a pair of spiral wraps of nylon monofilament is helically wound around the shell. The patent discloses that the core may be made of a variety of materials, such as nylon, polyester, fiberglass, and aramid fibers such as KEVLAR and NOMEX. However, without a protective wrap of abrasion resistant material around the core, in accordance with the present invention, notching of the conventional outer wraps disclosed in this patent can readily occur, and thereafter a NOMEX core alone (low in tensile strength) is not capable of bearing the load, resulting in string failure.
While the present invention can be understood and readily practiced by those skilled in the art without an understanding of the underlying theories of racquet strings, U.S. Pat. No. 4,183,200 to Bajaj, U.S. Pat. No. 4,565,061 to Durbin and U.S. Pat. No. 4,586,708 to Smith, et al. are cited herein as disclosing certain theories of what makes a good playable string, the disclosures of which are incorporated herein by reference. Bajaj has theorized that a constant spring rate (which measures the static stiffness or the elastic modulus) is the main contributing factor of a string's playability. Durbin has theorized that a good playable synthetic string should have a tensile stress greater than 20,000 psi and an elastic modulus less than twice the tensile stress, in contrast to what has been thought to be desirable as the opposite. A natural gut, for instance, has a tensile stress/elastic modulus ratio of 0.13, whereas the commercially available synthetic showed the ratio to be around 0.30. Basically according to Durbin's teachings, a string with a relatively lower elastic modulus or static stiffness, as disclosed in Bajaj, is preferred. Smith, et al. have theorized that for a racquet string to have good playing characteristics, it must possess several important properties, namely resilience (coefficient of restitution which measures the amount of energy which is returned to the ball by the string on impact) and elasticity (which measures the dynamic stiffness).
Smith, et al.'s string is composed of polyetheretherketone, also known as PEEK. Prince Manufacturing, Inc. utilizes this technology to produce PREMIERE strings which consisted of 100% PEEK coated with nylon. The PEEK string exhibited some increase in durability and notch resistance over conventional nylon strings. However, the string made of PEEK could not provide the superior combined properties of playability, durability and resistance to notching achieved by the string of the present invention. Prince Manufacturing Inc. also marketed a subsequent string called RESPONSE which was a combination of PEEK with nylon multifilaments. This string gave a small improvement in durability but at the sacrifice of playability and thus provided only a modest improvement in combined properties of playability, durability and resistance to notching.